Silicone pressure sensitive adhesives (hereinafter also referred to as PSAs) typically contain at least two primary components, namely a linear siloxane polymer and a tackifier resin consisting essentially of triorganosiloxane (M) units (i.e., R.sub.3 SiO.sub.178 units, in which R denotes a monovalent organic group) and silicate (Q) units (i.e., SiO.sub.4/2 units). Such "MQ" resins which have heretofore been used to prepare silicone PSA compositions generally have M/Q (molar) ratios in the range of about 0.6 to 0.9. Furthermore, the prior silicone PSA art known to applicants generally does not attribute any significance to the molecular weight of these MQ resins. Where the molecular weight is discussed to any extent, the prior art does not suggest any criticality thereof in connection with the above mentioned M/Q ratios.
Thus, EPO Publication No. 0,255,226 to Minnesota Mining and Manufacturing Company (Copley et al.) teaches silicone pressure-sensitive adhesives (PSAs) containing (a) a high molecular weight polydimethylsiloxane gum (or copolymer) and (b) an MQ resin having a polydispersity of .ltoreq.2.0 wherein the M/Q ratio is 0.5 to 1.0. In this disclosure, the inventors prepare narrow fractions of a commercial MQ resin (GE-545). When these resin fractions are used in place of the total resin in the formulation of PSA compositions, the former exhibit superior adhesive performance.
Likewise, where the prior art does disclose a broader range of the above mentioned M/Q ratio, it does not suggest the combination of a specific ratio range and a specific molecular weight range to obtain improved adhesive characteristics. For example, U.S. Pat. No. 3,929,704 to Horning teaches a silicone PSA comprising MQ resin, silicone gum and a curing agent, wherein the curing agent is a peroxide admixed with a plasticizer and an extender. While the Horning patent teaches the broad range of M/Q of 0.5 to 1.5, it is silent with respect to any molecular weight limitations to be placed on the silicone resin and the preferred range of M/Q is stated to be 0.55 to 0.75.
Moreover, the prior art silicone PSA compositions generally employ a resin/polymer ratio of about 40/60 to about 60/40 by weight and do not suggest the relatively high resin/polymer ratios contemplated in the instant compositions.